Over the past 9 years of the funding of this grant, we performed the first studies in human subjects quantifying the rates of glucose and fatty acid substrate cycles, and have performed a number of studies evaluating the importance of these cycles in metabolic regulation. In the past four years, we have focused primarily on the triglyceride-fatty acid (TG-FA) cycles. We have found the TG-FA cycle in which fatty acids are released into the blood from peripheral adipocytes, cleared by the liver, reesterified into triglyceride and secreted into the plasma as very low density lipoprotein triglyceride (VLDL-TG) and transported back to the adipocytes, to be of major importance in the matching of fatty acid availability and requirement for substrate oxidation under rapidly changing conditions, such as exercise. In the past, our efforts have been directed primarily at understanding factors controlling the lipolytic side of this cycle. In the current proposal we will focus more on the hepatic side of the cycle. Specifically, our goal is to clarify in human volunteers the relationships between hepatic very low density lipoprotein triglyceride (VLDL-TG) output, the uptake of free fatty acids (FFA) by the liver, hepatic fatty oxidation, and de novo fatty acid synthesis. We propose that the rate of VLDL-TG output is controlled in part not only by the rate of hepatic fatty acid uptake, but also by the extent to which they are oxidized. Further, we propose that the relationship between fatty acid oxidation within the liver and the channeling of non-oxidized fatty acids into TG is controlled by two related mechanisms: changes in the total rate of fatty acid oxidation, and changes in the fractional rates of beta oxidation vs complete oxidation via the TCA cycle. We propose that the total rate of hepatic fat oxidation is controlled by the rate of hepatic glucose uptake, whereas the rate of FFA uptake controls the proportion of fatty acids that are oxidized via beta oxidation, as opposed to TCA cycle oxidation. We will also investigate the role of de novo fatty acid synthesis in controlling VLDL-TG output. We propose that the incorporation of newly produced fatty acids into VLDL-TG is a direct function of the rate of synthesis of the fatty acids, and that de novo fatty acid synthesis does not directly affect the conversion of plasma FFA to VLDL-TG. Further, we propose that the primary determinant of the rate of hepatic fatty acid synthesis is the rate of hepatic glucose uptake, regardless of the amount of plasma FFA taken up by the liver or the prevailing insulin concentration. We will assess these hypotheses using a combination of stable isotope tracer techniques that the net splanchnic balance technique. Various experimental protocols will involve the control of plasma FFA, glucose and insulin concentrations.